Flow meters are widely used in industry and research to indicate and/or control flow rates of gases or flow rates of liquids to improve processing and increase the operation efficiency of the equipment involved. This type of instrument is basically a vertical-internally tapered, so-called "float" or meter tube mounted with the large end at the top of the instrument and a ball-like float or rotor with an outer diameter slightly less than the minimum diameter of the tube, being placed inside the float tube. The clearance space between the float and the float tube internal surfacing forms an annular passage or orifice, and as the float tube is defined by a tapering bore that diverges upwardly, the area of this orifice is larger when the float is near the top of the float tube than when the float is near the bottom of the float tube. By connecting the float tube into a fluid flow line so that fluid flow direction is from the bottom to the top of the float tube, the float will move upwardly and be supported at a point where the orifice is just large enough to pass the fluid flowing through the system. By making the float tube transparent so that the position of the float can be seen and by providing a scale along side the float tube, the flow position may be "read" in terms of numbers on the scale which are provided in accordance with standard practices in this field.
The rotameter has become one of the most popular flow meters in the chemical process industries. In the usual type of rotameter a weighted plummet or float contained in an upright tapered tube is raised to a position of equilibrium between the downward force of the flow and the upward force of the fluid flowing past the float through the annular orifice surrounding the float. The term "rotameter" was derived from the fact the plummets originally had slots to impart a rotational force thereto for the purpose of centering and stabilizing the float. The present trend, however, is toward non-rotating floats.
To provide for more versatility in the application, interchangeable precision-bore glass tubes and metal metering tubes are available. Rotameters have proved satisfactory for both gases and liquids at high and low pressures. A single instrument can readily cover a ten-fold range of flow, and by providing "floats" of different densities, a two-hundred fold range is practicable.
Aside from rotameters which require visual observation of the height of the float inside the tube, there exists electronic flow meter devices commercially available that operate on different principles such as rotating vanes, pressure drops, ultrasonic measurements, etc. Such commercially available electronic flow meter devices disadvantageously require that the liquid or gas flow to be measured contacts metal parts of the flow meter devices. Besides the problems associated with invasively contacting the fluid flow to be measured, known electronic flow meters generally are unable to operate in the presence of large magnetic fields such as NMR fields generally used in connection with medical environments including working hearts and other circulatory systems.
While automatic monitoring and control is desirable in flow metering, conventional rotameters used today generally require visual observation and manual recording.
U.S. Pat. No. 4,630,485 to Wastl, Sr. discloses a gas or low volume liquid flow meter permitting non-invasive electrical monitoring of the flow of a fluid at relatively low pressures and flow rates. The flow meter of this patent includes a housing provided with a generally cylindrical shaped chamber and a spherically shaped float indicator which is movable vertically within the chamber. A position detector means is also provided, non-invasively mounted to the flow meter, for detecting the vertical position of the indicator and providing an electrical output signal which is responsive thereto. The position detector includes a light source and a light sensitive element which is a photopotentiometer having a light sensitive surface. The resistance across the light sensitive surface of the photopotentiometer increases as the shadow cast by the float indicator moves upwardly.
U.S. Pat. No. 4,241,739 to Elson discloses a volume calculator for a spirometer which includes a vertically extending chamber and a movable indicator located with the chamber. The chamber is arranged between an energy source and an energy detector which communicate with one another by means of an energy beam, the chamber being transparent to the particular type of energy transmitted by the energy source. When the indicator rises to a height in the chamber adjacent the energy source, the energy beam is interrupted and the electronic circuitry is activated to measure the time that the energy beam is interrupted and calculates the total volume of inspired air.
The present invention provides for a means to accurately monitor fluid flow utilizing a standard rotameter. More particularly, the present invention provides a means for monitoring the position of the float in a rotameter in a manner that optically scans the rotameter for the float position.